1. Field of Invention
Silicon is, at present, the most important material in modern semiconductor technology and is finding increased use in solar cells for the photovoltaic generation of electricity. In view of the importance of the solar cell application, the stringent requirements for purity and low cost and further in view of the orientation of the work done, the process and apparatus is described primarily in the context of production of silicon for solar cell use. However, it is to be understood that both the process and apparatus used are generally useful in the production of silicon for whatever end use, as well as other transition metals such as Ti, Zr, Hf, V, Nb and Ta.
A major deterrent to the development of practical solar photovoltaic systems is the cost of high purity silicon. With todays technology, approximately twenty percent of the total cost of a silicon solar cell is ascribed to the silicon material alone. That is, the cost of the silicon material produced by the conventional hydrogen reduction of chlorosilanes constitutes at least twenty percent of the cost of producing the cell. It is estimated that the cost of the silicon must be reduced by almost an order of magnitude before silicon solar photovoltaic panels will prove to be economically feasible as a power source. The fact that the chlorosilane processes require multiple separations, which are so energy intensive and require such large capital investments, indicate that cost of the silicon cannot be reduced sufficiently to make silicon solar cells economically feasible without a major process change. As a consequence, an approach to the production of solar grade silicon which is less complex, less energy intensive and which requires less capital equipment is required.
2. Technical Field of the Invention
It has been found that silicon of more than sufficient purity to meet the solar cell applications can be produced within the economic requirements from the metallic reduction of silicon fluoride. Preferably, the silicon fluoride is prepared from an aqueous solution of fluosilicic acid, a low cost waste by-product of the phosphate fertilizer industry by treatment with a metal fluoride which precipitates the corresponding fluosilicate. This salt is filtered, washed, dried and thermally decomposed to produce the corresponding silicon tetrafluoride and metal fluoride which can be recycled to the precipitation step. The silicon tetrafluoride is then reduced by a suitable reducing metal and the products of reactions are treated to extract the silicon. Each of the steps is described in detail using sodium as typical reducing agent, and sodium fluoride as typical precipitating fluoride but the concept applies as well to other reducing metals and metal fluorides that can reduce silicon fluoride and form fluosilicates. The process in one form is described in detail in an article entitled Silicon by Sodium Reduction of Silicon Tetrafluoride authored by A. Sanjurjo, L. Nanis, K. Sancier, R. Bartlett and V. J. Kapur in the Journal of the Electrochemical Society Vol. 128, No. 1, January 1981 and the subject matter of that article is specifically incorporated herein by reference.